Optical discs, on which signals representing a huge amount of information can be stored at high densities, have recently been used more and more extensively in a number of fields of audio, video, computer and other technologies. And yet the storage capacities of those optical discs are required to be further increased, and a lot of people have taken various approaches to achieve that object. One of those approaches that have been adopted so far to further increase the storage capacity is to provide multiple information storage layers.
In such a multilayer optical disc, however, light reflected from a target storage layer to read information from (which will be referred to herein as a “read layer”) and light reflected from a storage layer other than the read layer (which will be referred to herein as a “non-read layer”) often interfere with each other to threaten the stability of servo control, which is a problem.
For example, if the interval between two information storage layers of a multilayer optical disc is relatively close to the magnitude of focus error represented by the amplitude of the S-curve of a focus error (FE) signal as defined between the maximum and minimum peaks thereof (which amplitude will be referred to herein as an “S-curve amplitude”), then the S-curve signal representing the light reflected from the read layer and the S-curve signal representing the light reflected from the non-read layer will superpose one upon the other. That is to say, the FE signal will be affected by the light reflected from the non-read layer. As a result, the objective lens sometimes cannot be controlled accurately and the light beam spot sometimes cannot be formed on the target storage layer.
In this case, if the S-curve amplitude were sufficiently smaller than the layer-to-layer interval of a multilayer optical disc, this problem could be overcome to a certain degree. However, it would be difficult to shift the focal point of the light to as close to the read layer as to establish a focus controllable state easily. In addition, during a read/write operation, the stability of the focus servo control would be easily endangered when the drive is subjected to some disturbance such as vibrations, which is another problem to solve. Nevertheless, if the S-curve amplitude and the layer-to-layer interval were both increased to some extent, then the distance from the surface of the optical disc to the deepest storage layer would increase so much as to produce an increased spherical aberration in an optical head that uses an objective lens with a large numerical aperture (NA).
To overcome these problems, Patent Document No. 1 discloses a technique that uses both an FE signal with narrow S-curve amplitude detected by an astigmatism method and an FE signal with broad S-curve amplitude detected by a spot size detection (SSD) method in combination. Specifically, the optical disc drive disclosed in Patent Document No. 1 uses an FE signal with broad S-curve amplitude detected by the SSD method to shift the focal point of the light to as close to the read layer as to establish a focus controllable state easily. Thereafter, by using an FE signal with narrow S-curve amplitude that is less likely to be affected by the non-read layer, a focus servo is done on each of those layers.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2002-190132